1. Field of the Invention
This invention relates to methods and apparatus for regulating combustion within furnaces to achieve more optimum furnace performance.
2. Description of the Prior Art
U.S. Pat. No. 3,602,487 relates to enriching blast furnace gas with natural gas to heat a blast furnace stove by utilizing stove dome temperature to control addition of natural gas during a firing cycle by comparing measured dome temperature against a time profile of desired dome temperature output by a function generator. The time profile of desired dome temperature defines a control set point. An error signal, defined by difference between the measured dome temperature and a desired temperature-time profile at any time, is provided to a natural gas flow control.
'487 also discloses monitoring stove dome temperature and adding air to blast furnace gas flowing through the stove to prevent stove dome temperature from exceeding a predetermined constant temperature.
U.S. Pat. No. 3,947,217 discloses producing inert gases and regulating combustion air drawn to the process where the regulating elements include a servo motor for positioning the elements fully open or closed where the servo motor is responsive to magnitude of signals succeeding each other and having intervals therebetween. Signal to a servo motor connected to the air flow regulator is provided in response to deviation of the measured value from a set point value and is proportional in magnitude to a difference in certain measured gas levels. Control signals are supplied only when upper or lower limits are exceeded, i.e. there is a deadband within which the control system does not operate.
Of lesser interest are U.S. Pat. Nos. 3,074,644; 3,224,838; 3,354,931; 3,503,553; 3,616,274; 3,861,855; 3,973,898; 4,032,285; 4,059,385 and 4,097,218.
One previous approach in attempting to achieve relatively optimum operation of fired furnaces has been to measure oxygen content of flue gas and to control air input to the burner in response thereto. This has not been entirely successful. The excess air at which the furnace burner operates at greatest efficiency must be known, however, the excess air-efficiency characteristic varies as furnace burn rate varies, so that air input to the burner cannot be maintained constant. Changes in fuel in the furnace also change the excess air-burner efficiency ratio. As an additional difficulty, air drawn into the furnace after combustion can provide an erroneous indication of burner efficiency because oxygen analyzers cannot differentiate between excess air provided to the furnace burner and air leaking into the furnace.
Another approach to maximizing burner performance in fired furnaces has been to optimize air flow based on level of carbon dioxide in the furnace flue gas. This approach has not been entirely successful. Leaks in the furnace cause air dilution of the flue gas with consequent reduction in the percentage of carbon dioxide. Moreover, fuel and firing load changes may alter the carbon dioxide percentage in the flue gas. Finally, even if a desired carbon dioxide content set point is provided, a controller comparing measured carbon dioxide against the set point value gives no indication as to whether air flow should be increased or decreased so as to make the measured carbon dioxide percentage more closely approach the set point percentage.